1. Field of the Invention
This invention relates to a clock test apparatus for an electronic device such as a receiver, for modeling clock error in a clock generator for use in the receiver, and analyzing and checking the performance of the receiver by simulating the clock error.
2. Description of the Related Art
As is well known, a GPS (Global Positioning System) receiver is mounted in a moving body such as an aircraft or a ship, receives radio waves from GPS satellites, and outputs positional data. The GPS receiver also detects the Doppler frequency of the radio waves by means of a clock, and calculates the velocity of the moving body relative to the satellite from the detection result. A navigation calculation is performed on the basis of the positional data and the relative velocity. Hence, the performance of a GPS receiver greatly depends on the clock characteristic for detecting the Doppler frequency. Accordingly, to analyze or check the performance of a GPS receiver, it is necessary to model and simulate a clock error in the clock generator, thereby obtaining design data.
Further, a Kalman filter is used to perform the navigation calculation on the basis of the relative velocity, thereby increasing the accuracy of the calculation. The Kalman filter has a clock error dynamics model of the GPS receiver. In a GPS receiver, since the clock error dynamics model is an essential feature, the property of the model influences navigational accuracy.
In the conventional art, short-term stability, which is statistically obtained from the average value of clock frequencies, is used as an index of the characteristic of a clock, and the performance of a GPS receiver is analyzed and checked on the basis of the short-term stability. However, since the short-term stability is a statistical index, how the clock fluctuates in practice as time passes and the status of the clock error dynamics (a differential equation representing a clock error dynamics) are not taken into account. Hence, the performance of a GPS receiver cannot be analyzed and checked reliably on the basis of short-term stability.
Although the above description relates to a GPS receiver, the same problems arise in other electronic apparatuses incorporating clock generators.